In recent years the domestic oil industry has been placing greater and greater emphasis on the secondary (and tertiary) recovery of oil. This is so because (1) exploration costs have increased, (2) large domestic reserves are rarely found, and (3) crude oil imports are required to meet demands. These reasons coupled with ever more stringent environmental protection requirements have resulted in a continual domestic crude oil shortage.
Waterflood, the oldest assisted oil recovering method, remains the most commonly used method in the United States for increasing domestic supplies. In waterflood, water is introduced into an oil reservoir through an injection well. As used herein an injection well is a well utilized in secondary recovery of petroleum, in which a fluid such as gas or water is injected into an oil reservoir to provide supplemental energy to drive the oil remaining in the reservoir to the vicinity of production wells.
In the practical application of injection wells, a new well may be drilled or a producing well may be converted to an injection well. An injection well therefore includes the injection tubing string in addition to the packer, casing, and possibly a production casing. In operation of an injection well, it is generally impossible to make a statement as to the optimum injection rate for flooding a reservoir because of the wide range of rock and fluid characteristics in oil reservoirs. It is recognized, however, that excessive injection pressure, which can lead to uncontrolled reservoir fracturing, should be avoided. On the other hand the injection rate and the accompanying injection pressure must be sufficient to maintain production.
A problem that arises with injection wells is that sometimes the water level rises into the borehole and fills the space between the various tubes and the casing. In active wells a leak in the injection tubing or packing can fill the annulus between the injection tubing and the casing, and injection water will eventually seep into the formation surrouding any leak in the casing. Another possible scenario is that a leak in the injection tubing or packing, which allows injected water to back up into the annulus, would be accelerated by an excessive injection rate. Water levels can also rise in inactive wells due to leaks in the casings.
In the event of a leak in the casing, injection water can seep into the surrounding formation and contaminate underground sources of drinking water which are at depths below the leak in the casing. Consequently the United States Environmental Protection Agency has enforced mechanical integrity regulations for water injection wells. These regulations require non-leaking casings so as to safeguard the underground sources of drinking water.
Since casing leaks are expensive to repair, and because injection wells having casing leaks are especially numerous in older oil fields where extended periods of waterflood have been employed, a method for determining the presence of water at a level in the borehole of an injection well which would not threaten underground sources of drinking water, and which would allow time for steps to be taken to prevent further rise of water in the borehole, would be highly desirable. The detection method would thereby safeguard underground sources of drinking water without making expensive repairs necessary.
Accordingly, it is an object of this invention to provide apparatus and method for detecting the presence of a liquid level in a borehole which is reliable, inexpensive, and relatively simple to implement.
It is a further object of this invention to detect the presence of liquid in a borehole without employing electrical devices in the borehole.
It is still another object of this invention to protect underground sources of drinking water from contamination by injection water used in secondary oil recovery.